The orderly balance of cell division, tissue repair, and programmed cell death is essential for the maintenance of health in multicellular organisms. Many clinically important diseases, such as cancer, inflammatory diseases and neurodegenerative diseases such as Alzheimer's, Huntington's and Parkinson's disease result in from an imbalance between mitotic reparative and apoptic events. In health, decisions controlling cell differentiation, mitosis, and death involve exquisitely regulated signal transduction pathways linking kinases and effector molecules through adaptor molecules and molecular signaling domains. Microbial and chemical toxins, genetic mutations, or viral mimicry of signaling molecules inappropriately activate these pathways to cause human diseases and tumors. Mitotic and apoptic signaling pathways are regulated through protein phosphorylation on serine, threonine, and tyrosine residues. Details of molecular events following serine/threonine phosphorylation, in contrast to tyrosine phosphorylation are unclear. However, Dr. Yaffe and others have recently found that a variety of protein-protein complexes are formed through direct phosphoserine-mediated reactions, much like phosphotyrosine interactions seen in complexes involving SH2 and PTB domains. Many of these signaling complexes involve a master regulatory protein called 14-3-3, which regulates essential mitotic and apoptotic signaling molecules through phospho serine-dependent binding. 14-3-3 molecules are ubiquitous and evolutionarily conserved, increase in amount following injury and DNA damage, and are targeted by pathogenic viruses, yet their function remains mysterious. Dr. Yaffe believes that different serine/threonine kinases generate unique subsets of the general phosphoserine motifs bound by 14-3-3 to target and regulate distinct downstream effector molecules in response to the physiological state of the cell. The number of 14-3-3 binding sites within a particular kinase target, and the manner in which both the phosphorylated motif and the remainder of the protein interacts with 14-3-3 then determines whether 14-3-3 functions as an adaptor, scaffold, chaperone, or sequencing protein for that particular effector to control subsequent downstream signaling events. Dr. Yaffe proposes a combination of combinatorial peptide library experiments, functional genomic screening, and structural biological studies to elucidate this kinase- dependent binding, identify kinase specific targets, and uncover the structural basis for 14-3-3 function. These studies fit within the Applicant's long term objectives to understand the cellular, biochemical, and structural basis for phosphoserine dependent signaling.